There is a progressive increase in the general background level of mercury in our North American environment. Inorganic mercury in the environment can be converted to organic mercurials in the biological food chain and accumulate in food stuffs, principally as methylmercury. Man may directly consume methylmercury as an occupational hazard or may indirectly acquire chronic intoxication as the final predator in the biological food chain. Methylmercury in man, unlike inorganic and some other organic mercurials, is widely distributed in body tissues, producing a different pattern of clinical findings than inorganic or elemental mercury. The cerebrum and cerebellum are principally affected. Methylmercury freely passes through the placental barrier of man and some experimental animals. In addition to the neuronolysis seen in the adult, the fetal brain, injury is diffuse. A generalized hypoplasia of the brain and other viscera are seen. Excessive defective mitoses and chromosome breakage has been found in human lymphocytes cultured from patients with methylmercury poisoning. Beyond this fragmentary evidence little is known about the teratogenic effects of mercury on the primate. The investigation is designed to uncover deleterious effects on the fetus at the level of human "subclinical" exposure as well as at higher dosage levels. The purpose of this experiment is to determine the threshold level of chronic exposure of pregnant monkeys to methylmercury which produces neurophysiologic deficits in the offspring. Other possible teratogenic parameters in the primate such as teratogenic dose range, pattern of anomalous development and embryonic period of susceptibility will be recorded. Although rodents are an excellent experimental animal for determining some features of teratogenesis, extension of the studies to primates will yield more information more directly transposable to man. These experiments are designed to determine whether chronic low level doses of methylmercury produce subtle sensory behavioral and intellectual changes; parameters which are difficult to assess in lower animals.